This invention relates to an improvement in delta modulation techniques. An object of the invention is to reduce accompanying quantization noises by this technique.
Generally, delta modulation methods include a linear type delta modulation method and an adaptive type delta modulation method. FIG. 1 shows the adaptive type delta modulation method and FIG. 2 shows the signal waveforms of an input signal, output signal and modulation signal of such a method. For a quick understanding of this method, the integration step size, i.e., a variable value by one step of integration, for the following description of the invention, is assumed to be constant, but the integration step size is of course moderately affected by the adapted delta modulation method. The integration step size is determined on the basis of the compression or expansion characteristic by the slope and/or amplitude of an input signal. Since the delta modulation method at the receiving side provides a local demodulator which is the same as that provided at the sending side, an output waveform of the local demodulator, in this description, is substituted for a demodulation waveform obtained at the receiving side.
Referring to FIG. 1, element 1 is an analog comparator; element 2 is a holding circuit (usually a D flip-flop), element 3 is a syllabic filter and integrator; terminal 4 is an analog signal input; terminal 5 is a clock input where the clock determines the hold-timing of the holding circuit 2; terminal 6 is the output of the holding circuit 2, and is a digital signal output of the delta modulator; and line 7 is the output of the syllabic filter and integrator 3, i.e., the output of local demodulator 8. The delta modulator is so looped that the output of the integrator 3 tracks an analog input signal provided at input 4 and coresponding to the clock timing, whereby the output 7 of integrator 3 becomes the analog output of the receiver. A digital output becomes the transmitted signal and the local demodulator 8 itself is used at the receiving side.
The analog signal e.sub.in, when fed to the analog comparator 1 via input 4, is compared with the output e.sub.out of the local demodulator 8; at which time, if e.sub.in &gt;e.sub.out, then the output of the analog comparator 1 becomes V (H), and if e.sub.out &gt;e.sub.in, then it becomes V (L). The output of comparator 1 is the input of the holding circuit 2, and is held by the clock from clock input 5 and integrated so as to be fed to the .crclbar. input of the comparator 1. Hence, e.sub.out tends to approach e.sub.in at every clock.
The syllabic filter serves to change the integration stepsize of the signal provided to the integrator 3 in order to provide the compression and expansion characteristic in dependence upon the slope and amplitude of input signal, thereby preventing slope overloading. Slope overloading refers to the condition that the demodulation waveform departs considerably from the input signal, because an amount applied to the integrator 3 cannot reach the input signal.
A signal from the digital signal output 6 of the delta modulator passes through a transmission line to reach the input of the receiver (i.e.--the input of the holding circuit 2). Since the holding circuit 2 holds the input signal in response to clock signal 5, the output of syllabic filter and integrator 3 becomes equal to that of the input signal except for being delayed by one clock.
FIG. 2 shows each signal waveform of the delta modulator, in which a is a waveform of the analog input signal, b is that of the clock signal for the holding circuit, c is that of the digital output of the delta modulator, and d is an output signal from the local demodulator, and also represents the demodulated waveform at the receiving side, which, if necessary, is subsequently passed through a low-pass filter.
The waveform at each unit of the conventional delta modulator is shown in FIG. 2, in which no slope overloading is generated. In other words, the adaptive type delta modulator is constructed to eliminate the slope overloading, whereby both the linear type delta modulator and the adaptive type delta modulator are equally effective in generating no slope overloading.
The speed of changing a time constant of the syllabic filter at the adaptive delta modulator, and in turn, a step size of integration, largely contributes to the characteristic response of the circuit. When a large time constant is used, the step size is smaller than needed and the system response becomes poor. On the other hand, when a small time constant is used, the system response is improved, but the step size is larger than needed.
As seen from FIG. 2, the waveform d tracks the input waveform a corresponding to the timing of clock b. In other words, the digital output c is integrated, so that a is integrated in plus when the digital output c is H and in minus when it is L. Waveform e of FIG. 2 shows the difference between the analog input waveform a and the demodulation output d.
Quantization noise which is the difference signal e, is incidentially generated in the delta modulation.